The invention concerns satellite navigation systems; these systems, using a constellation of satellites transmitting known signals, enable a receiver to compute its position within a geocentric frame of reference. These systems are used to navigate vehicles of all kinds, in particular ships and aircraft. The systems in use at present are the GPS and GLONASS systems, which comprise constellations of satellites at an altitude of 20 000 km. These systems are based on measuring pseudo-distances between the satellites and the receiver. A pseudo-distance is evaluated from the transmission time of a signal transmitted at a particular moment by the satellite and received by the receiver. For this purpose, the receiver measures the arrival time of the encoded signal transmitted by a satellite of the constellation and reads the time at which the signal was transmitted by the satellite in the received message. The receiver evaluates the transmission time by determining the time the message was received or by computing the phase of the signal.
Knowing the position of the satellites makes it possible to determine the position of the receiver by triangulation on the basis of the pseudo-distances. Existing prior art systems use four satellites to compute the position of the receiver; four measured pseudo-distances are thus used to determine the time and the position of the receiver station.
One of the main features of the above systems is that there is no limit on the number of users, in other words the system cannot be saturated; the satellites merely transmit and position is computed at each receiver. The receiver is therefore passive, operating in "listening" mode. This also implies that the pseudo-distances are measured on the one-way satellite-to-receiver path. Other details on the structure and operation of the above navigation systems can be found in the prior art documents cited below.
U.S. Pat. No. 5,420,592 describes the use of a GPS navigation system for positioning meteorological balloons; it proposes separating the GPS receiver into a mobile part carried by the balloon and a fixed part on the ground. A Kalman filter is used to predict the position of the balloon and enables positioning even with reduced visibility of the constellation of satellites. The system corrects the offsets induced by the Doppler effect in the signals from the satellites to enable correction of the signal from the satellite for fast computation of the pseudo-distance.
EP-A-0 460 862 describes a GPS type global positioning system receiver that can be implemented in a VLSI circuit. The receiver includes means for evaluating the pseudo-distance and the variation in the pseudo-distance in order to eliminate the Doppler effect from the carrier from the satellites.
U.S. Pat. No. 5,436,632 proposes to use the Doppler effect to compute corrections to be applied to the rate of variation of the pseudo-distances in a system for monitoring the integrity of the signals from a GPS type system.
U.S. Pat. No. 5,343,209 proposes to evaluate the Doppler shifts using an estimate of the speed of the receiver and the variations of the line of sight (LOS) of a GPS type positioning system. That evaluation produces an estimate of the phase of the carrier, the frequency of the carrier and the phase modulation of each of the received signals which is better than could be provided by each signal independently.
EP-A-0 518 146 describes a multi-point positioning system for a ship and towed buoys. That system uses a plurality of GPS receivers and Kalman filters to evaluate the quality of each position indication. It is again proposed to correct the Doppler shift for evaluating the pseudo-distances.
U.S. Pat. No. 5,594,453 describes a GPS receiver with a waiting mode to reduce power consumption; in order to enable rapid acquisition of the position on leaving the waiting mode, that document proposes to correct the influence of temperature on the clock frequency. The Doppler effect is used to compute the estimate of the frequency of the satellite in the new position, on leaving the waiting mode.
U.S. Pat. No. 5,557,284 proposes a system for detecting spoofing (the transmission of fake signals) in a global positioning system; it proposes to use the differences in the Doppler effect between two receivers on an aircraft or on the vehicle to detect fake signals.
In all the above documents, as in the prior art in general, the Doppler effect embodied in the signals from the satellites is seen as a drawback; it is proposed to correct the signals received to eliminate the Doppler effect or to allow for the Doppler effect in predicting the position of the receiver. None of the above documents proposes to use measurement of pseudo-speeds by the Doppler effect to improve the accuracy of the position determined. Also, all the above documents concern constellations of satellite in medium Earth orbit (MEO), such as the GPS or GLONASS systems.